Evolution oF Thrusting

The overall evolution, therefore, is for thrusting to begin at deeper levels and to progress upward and outward (i.e., away from the root zone) from a major sedimentary basin. Ductility contrast in the stratigraphic section encourages a stair-step evolution of thrusts, which we know as ramping. A relatively flat portion of a thrust plane, particularly where it remains parallel to bedding within a single lithology, is referred to as decollement. If this flattening of the thrust plane occurs at the boundary between the sedimentary section and basement, it is called basal decollement.
The progressive stacking of thrust faults may develop in piggyback fashion, where younger thrusts form in the footwall or, alternatively, in overstep fashion, where thrusts become younger toward the root zone. Piggybacking seems prevalent on a regional scale and is by far the more significant progression. At the same time, both piggyback and overstep thrusting occur on a more local level, often as a result of imbrication.
Imbricates occur most often in two structural positions of high stress concentration-near the toe of a major thrust and above ramps in a thrust plane. They dip steeply as they approach the surface, and stack slice after slice of the same stratigraphic section along faults, which sole out into a major thrust plane. Continued movement along this plane after the imbricates have formed will rotate them, so they can become vertical and overturned.
As discussed by Dahlstrom (1970), imbrication actually offers a basic model for foreland thrusting-as the scale of a cross section is increased to become more regional, major thrusts themselves become imbricates of the largest faults (i.e., those with the greatest displacement). These, in turn, can be thought of as subsidiary faults to a basal detachment or decollement plane that marks the structural boundary between basement, usually crystalline metamorphic or plutonic rocks, and sedimentary cover.